DIY Mic Amp- Clean, Transformerless, Battery Powered, Discrete, Sonosax-esque.

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dlozanothornton

Active member
Joined
May 24, 2015
Messages
29
Location
Nova Scotia, Canada
Hi there,
I want to build a portable mic preamp, for condenser microphones only (Schoeps, Sennheiser, etc., so expecting 50 - 200 ohm impedence approximately - so 2.5 - 5k input impedance preferred).
The gain needs to only be from 20-50db of gain.
I want to design something with at least 15 - 20k frequency response within 1 db or better, discrete preferred but IC possible.
I want to make it run on a some rechargeable 9v batteries (up to 6) like the Sonosax or Sonodore designs (SX-M2 for example), but I feel lame just buying stuff and want to continue to modify it over the years.
Sonosax, SQN, Sonodore and the upcoming Pueblo Isleta are all very high quality designs with similar briefs.
High quality and linear are my primary goals here.
Could anyone point me in the right direction for this in any way?
I'll try and upload some possible schematics of my own at some point soon when I get a chance for critique, but I am fairly inexperienced, but I am not that interested in DIY-ing coloured preamp designs.
Maybe there is a DIY project out there already that covers most of this but could adopted to battery work. I am looking through the archives atm...
 
There are a number of off the shelf mic preamp ICs that are very linear and quiet if built following their app notes.  No need for heavy design lifting like decades ago.

JR

PS: Check THAT corp and TI offerings to start...
 
Just thinking out loud, you could take two 24V rechargeable batteries and stack them to make +-24V. Simple RC + capacitance multipliers to knock that down to +-15V and you have a great supply for an IC amplifier. Connect -24V to pin 1 of the XLR and chassis and +24 to pins 2 and 3 to send 48V to the mic. Obviously you cannot connect the 0V IC ground to pin 1 / chassis but that's fine because if you use an IC like THAT 1512 you need blocking capacitors anyway (see Figure 5 from THAT 1512 datasheet). Then just do an impedance balanced output with the 1512 directly or, if you want to drive long cables, use a THAT 1646 (also with blocking cap on the output since 0V is "virtual ground"). Use a steel enclosure with proper grounding / RF filtering. That could be made very solid.
 
I'd probably start with a split supply, four * 9V batteries, and run with That 1512's. Headroom really depend on what you are recording with, there is no point in getting +24dB out if your recorder can't handle it without padding down. I'd also have a highpass filter. What about P48 supply for the mics?. Another option on the power front would be a lovely LT1533 from linear/analog.com that would make DC in easier to handle. I'd check out some of the film/location sound guys for ideas (but you probably have)
 
Is there any need to do anything like current regulate or filtering with a battery supply for this sort of thing?
Especially for the phantom supply?
I was thinking 6 x 9v's because it would simplify the circuit for getting phantom (without resorting to more complicated charge pump circuits which could introduce weird noise). Running it off 3 AA's with a 1533 or similar would be slightly more..friendly. Hmmm.
It won't have to drive anything longer than 1-2m of cable so it looks like I could go for a differential mic preamp like That 1580 (any, similarly high fidelity, alternatives??) and take the input to the AD right from the outputs of that. Looks like it has the lowest THD of THAT preamps and sends a differential out direct, possibly mitigating the need for another IC stage or transformer.
So I could just copy the THAT 1580 circuit, point to point wire it on a turret board with metal film resistors (+/-1% or better), fancy Wima or Mundorf oversized capacitors and an Elma stereo switch or something, steel enclosure, figure out how to supply the phantom power and I should be on to an easy winner.
Hi-pass filter is a good idea, that could be done with a switch of cap at the gain switch right?
Right?
Very helpful suggestions people :) I'm a big researcher but I'm really throwing around ideas I don't fully grasp, so I appreciate this a lot.
 
Hi,

I built a battery powered mic preamp a few years ago for recording birdsong.
See my comments here ....
https://groupdiy.com/index.php?topic=66749.0

I was using AKG mics with a phantom requirement of 9 to 52V, so this gave simpler supply options.
The circuit was based on THAT mic preamp chips closely following data sheet suggestions.
Worked well for this purpose.
 
Pardon the promotion. We’ve sold quite a few Eden mic pre’s for portable systems. Quite a few customers have had success running from 2x 9V batteries.
 
Rochey said:
Pardon the promotion. We’ve sold quite a few Eden mic pre’s for portable systems. Quite a few customers have had success running from 2x 9V batteries.

Thanks, looks like a good and cheap option. I think I would prefer to make something from scratch, I like swapping out components and testing stuff and I have a lot of time.
 
So this seems like a fairly standard, AC-coupled, RFI protected, phantom always on, no pad, input stage. I don't fully understand the effect of C1 and C2 (I know to block off the phantom DC) on the low pass cutoff frequencies and are R3 and R4 just nice and small as current limiting resistors?
This is redrawn verbatim from the THAT 1570 instructions.
Would this even be a perfectly functioning phantom power supply as is?
I wonder could this be the input stage of a BJT or JFET preamp?
 

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dlozanothornton said:
So this seems like a fairly standard, AC-coupled, RFI protected, phantom always on, no pad, input stage. I don't fully understand the effect of C1 and C2 (I know to block off the phantom DC) on the low pass cutoff frequencies and are R3 and R4 just nice and small as current limiting resistors?
They insure that 470pf LPF works, "and" provide some current limiting if  47uF caps charged up tp 48V gets grounded driving -48V into preamp IC.
This is redrawn verbatim from the THAT 1570 instructions.
Would this even be a perfectly functioning phantom power supply as is?
I wonder could this be the input stage of a BJT or JFET preamp?
Most such preamps are BJT to present good match to typical microphone source impedance (150-200 Ohm).

JR
 
They insure that 470pf LPF works, "and" provide some current limiting if  47uF caps charged up tp 48V gets grounded driving -48V into preamp IC.

Is it worth putting  10uH or so across the 10R's to improve RF  protection, or just  a CMRR inductor on the mic in just after the XLR?
 
Martin Griffith said:
Is it worth putting  10uH or so across the 10R's to improve RF  protection, or just  a CMRR inductor on the mic in just after the XLR?

Simple RCs are (were) enough for most cases.  I guess there is more RF around these days. I've seen some slick dual chokes used right at the input jack, for problem installs. Maybe that is what you are describing as a CMRR inductor?

JR 
 
Yep, a core that takes both mic signal leads, for common mode,. But I was wondering about putting individual inductors as well on each IC signal in pin. I'm thinking of the 2.4GHz burps. Mainly for RF bullet proofing the inputs, as I assume RF garbage is only going to get worse in the future.
 
dlozanothornton said:
aomahana- really intriguing, any links to the album? I'd love to hear it..

There are some mp3 excerpts on my website ....
www.audioarts.co.nz
The whole album is only available as a CD rather than a download.
The audio resolution is important to be able to enjoy finer details of the sound environment.
 
Note that if you're thinking about actually using the THAT 1570 / 1580, that is a tiny QFN package that is only 4mm across total. You'd have to be Ant-Man to solder that.
 
> Is it worth putting  10uH or so across the 10R's to improve RF  protection

Won't improve the RF--- it is still 10 Ohms.

It "does" take 20 Ohms out of the audio path. Hardly worth considering. "May" increase susceptibility to DC accidents. 
 
squarewave said:
Note that if you're thinking about actually using the THAT 1570 / 1580, that is a tiny QFN package that is only 4mm across total. You'd have to be Ant-Man to solder that.
damn, thats not happening. I guess 1510 it is, or discrete.
Re-reading my copy of The Art of Electronics to try and figure out how to make a BJT based mic preamp.
I could actually go unbalanced out, possibly further simplifying the design..
How do I make a really high quality BJT based mic preamp with modest gain that only needs to serve known impedance condenser mics? Any links or books?
 
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